Gently does it!: in situ preparation of alkali metal-solid electrolyte interfaces for photoelectron spectroscopy.

The key charge transfer processes in electrochemical energy storage devices occur at electrode-electrolyte interfaces, which are typically buried, making it challenging to access their interfacial chemistry. In the case of Li-ion batteries, metallic Li electrodes hold promise for increasing energy and power densities and, when used in conjunction with solid electrolytes, the adverse safety implications associated with dendrite formation in organic liquid electrolytes can potentially be overcome. To better understand the stability of solid electrolytes when in contact with alkali metals and the reactions that occur, here we consider the deposition of thin (∼10 nm) alkali metal films onto solid electrolyte surfaces, where the metal is thin enough that X-ray photoelectron spectroscopy can probe the buried electrode-electrolyte interface. We highlight the importance of in situ alkali metal deposition by assessing the contaminant species that are present after glovebox handling and the use of 'inert' transfer devices. Consequently, we compare and contrast three available methods for in situ alkali-metal deposition; Li sputter deposition, Li evaporation, and Li plating induced by e- flood-gun irradiation. Studies on both a sulphide solid electrolyte (Li6PS5Cl), and a single-layer graphene probe surface reveal that the more energetic Li deposition methods, such as sputtering, can induce surface damage and interfacial mixing that are not seen with thermal evaporation. This indicates that the appropriate selection of the Li deposition method for in situ studies is required to observe representative behaviour, and the results of previous studies involving energetic deposition may warrant further evaluation.

Interfacial interactions between CoTPP molecules and MgO(100) thin films.

We have investigated the interactions between cobalt(ii)-tetraphenylporphyrin (CoTPP) molecules and MgO(100) thin films on Ag(100) by means of Synchrotron Radiation X-Ray and Ultra-Violet Photoelectron Spectroscopy (SR-XPS and SR-UPS). At room temperature, the CoTPP monolayer consists of two different species. A minority of molecules exhibits a strong electronic interaction with the substrate, whereas for the majority a similar spectroscopic signature as for multilayer molecules is observed. Based on the lateral inhomogeneity of the surface electronic structure, we tentatively suggest that the strongly interacting molecules adsorb with their metal center directly above oxygen ions. Unlike for metal substrates, where a monolayer can be prepared upon heating to above 500 K, most of the monolayer on MgO desorbs at 550 K together with the multilayers. This indicates either a weaker molecule-substrate bond than for most metal surfaces or a higher activation energy barrier for dehydrogenation. The remaining molecules are presumably MgTPP molecules, originating from a 2HTPP impurity in CoTPP.